1. Field of the Invention
This invention is in the field of wireless radio frequency devices and methods of communication therefor and, more particularly, is an improved integrated circuit transponder device and a method of communication therefor.
2. Description of the Related Art
Integrated Circuits (hereafter "ICs") are, of course, well known to those skilled in the art of electrical engineering. Typically, one will have a plurality of ICs and other electronic components interconnected to form an electrical system for performing one or more functions. In most electrical systems, ICs are physically connected by way of a number of external conductors such as wires, thereby permitting each IC to communicate with different ICs or other electronic components in the system. Alternatively, those skilled in the art are familiar with methods of communicating with ICs without the use of interconnecting, external conductors such as wires.
In particular, one could form electronic packages enclosing both an IC die and an inductive coil with the IC's encapsulating material. These packages would have no conductors running externally from the package for the purpose of communicating with ICs or other electronic components outside of these packages. Such an externally wireless package encapsulates both the IC die having the chip logic, and the inductive coil, which is internally coupled to the IC die. The internal inductive coil of an externally wireless package generally serves two primary functions. First, when an electromagnetic field is applied through the package from an external source such as an external electromagnetic transmission source, a potential is created across the internal inductive coil, and this potential provides current to power the internals of the package. Second, the inductive coil serves essentially as an antenna for transmitting from and/or receiving information for the package internals. Such externally wireless packages may be referred to as transponders, meaning that they can both transmit and receive information.
Oftentimes, transponders are referred to as "tags." The term of art, tag, generally refers to externally wireless electronic packages that are affixed or "tagged" onto merchandise, luggage, or any one of a number of other objects where item identification is required. It should be pointed out that tags are not only used for item identification; they may also be used to rapidly report the state of some parameter regarding a particular item. For example, a tag having a temperature or pressure sensor could report the temperature or pressure of the item associated with the particular tag. Once a tag is affixed to a given item, it is read with a device typically referred to as a "reader," which sends out a certain excitation frequency that is detected by the tag, and then, when required, responded to by the tag. The reader can read data from a tag by detecting perturbations in the electric field caused by a tag transmission.
One variety of tags would be very simple. Such a simple tag would be idle when there is no reader field present, and when a reader field is asserted, then the tag transmits its data (e.g., item serial number, model number, etc.) continuously until the reader field is turned off. Alternatively, one might require a tag of greater functional capability. Creating a tag of greater functionality, yet limiting its production cost, generally implies the creation of a tag having minimum on-chip logic, and a reader with greater functional capability. In this manner, the reader can prompt different actions from the tag simply by sending different commands to the tag. This approach effectively yields a tag capable of executing more functions, prompted by different reader commands, with minimum cost of on-chip tag logic. Since the interface between a reader and a tag is usually air, it is very limiting. In other words, wireless communication between two elements such as a reader and a tag is relatively more difficult than communication between two other elements over an electrical conductor such as a wire. In light of the more challenging aspects of wireless communication, the communications link between a reader and a tag needs to be sophisticated enough to be able to pass commands back and forth between the two.
One possible manner of communicating with a tag would be to have a reader send actual commands to the tag. Such an approach would require complex tag circuitry to enable it to discern between an "empty" electromagnetic field, which is one devoid of any command for the tag other than a simple query for identification data from the tag, and a "loaded" electromagnetic field embedded with one or more commands for a tag. Moreover, different electromagnetic field strengths transmitted from a reader would necessitate the use of many high gain amplifiers on a tag to enable it to accurately detect commands on top of a carrier signal likely to be changing in orders of magnitude. Readers typically have high gain amplifiers, but placing such high gain amplifiers on a tag would require the tag to supply them with levels of power that a tag simply cannot deliver. Thus, if sending a variety of different commands to a tag is prohibitive because of tag power supply limitations, then a preferred communication technique would simply be to interrupt the electromagnetic field transmitted from the reader for a relatively short period of time in order to signal to the tag that it is then to commence a particular operation.
This interruption in the transmission of electromagnetic radiation from a reader to a tag is referred to as a "gap." In the simplest mode of operation, it is desirable that a tag be able to detect a single gap in the transmission from the reader in order to trigger a particular operation by the tag. However, in order to have a tag with greater operational capability, it must be able to detect multiple gaps in the electromagnetic field transmitted by the reader. More specifically, more capable tags will be able to recognize certain combinations of gaps established over certain time intervals, thereby triggering different operations from the tag. It is important to recognize that, in general, tags are powered only by the electromagnetic field inducing potential across the tag's internal coil, thereby producing current for the tag's internal logic. Now, it is also important to recognize that the internal tag circuitry has associated with it some capacitance capable of retaining some power for the tag logic. However, the power retaining capability of most tags is very limited, and therefor, it is necessary to refresh the tag's capacitance-retained power by recurring application of the reader's electromagnetic field. In order for a tag to be able to detect combinations of gaps, the reader's electromagnetic field must be applied, interrupted, and reapplied a number of times over particular time intervals. However, because the tag's capacitance-retained power supply must be regularly refreshed from the reader's electromagnetic field, the duration of any gap must be relatively short.
The previous discussion under the heading, "Description of the Related Art" is largely background information regarding certain aspects of the operation of a reader and a tag. Regarding the state of prior art tags, they detected gaps by determining when the potential at both ends of the tag's internal coil would be at or near chip ground. The term chip ground refers to a ground signal produced by the electronics on the IC die internal to the tag. More specifically, certain components of the tag's IC die process the signals coming from each end of the tag's coil in order to produce a ground signal for use by the rest of the internal tag circuitry. When prior art tags detected the condition where the potential at both ends of the tag's coil attained, or at least approached, chip ground at the same time, the tag circuitry detected a gap.
One problem with detecting this condition was that the coil voltages were measured relative to chip ground. The tag's internal circuitry must run off of some ground (i.e., chip ground) derived from the tag's coil, and that ground level depends on many factors. For example, when the tag is in a reader's electromagnetic field, the chip ground level is somewhat predictable because there is a rectifier in the tag's internal circuitry which references chip ground to the lower potential of the two coil terminals. Herein lies the problem. When the amplitude of the coil voltages goes to zero, the aforementioned rectifier essentially becomes an open circuit, and the coil is floating with respect to the chip ground. Of course, the capacitance of the tag's internal circuitry helps to maintain the chip ground signal; however, the reality of the situation is that when the reader's electromagnetic field is interrupted, and the coil voltages go to zero, the rectifier becomes an open circuit. Consequentially, the chip ground signal is too unpredictable to be effectively used as a reference to measure the potential of the coil terminals. As a result, the detection of a gap is hampered under the prior art approach.
Therefore, there existed a need to provide an improved tag capable of detecting a gap in a reader's electromagnetic field independent of the chip ground signal.